This invention relates to friction clutches having a plurality of interengaging friction elements alternately coupled to relatively stationary and movable clutch members and movable toward and away from each other along an axis.
A friction clutch of the above type is described, for example, in German Offenlegungsschrift No. 19 25 957. In such clutches, the magnitude of the torque transmitted depends, among other things, on the total number of cooperating friction elements, which may have disk-shaped or conical friction surfaces, on the mean diameter of the friction surfaces of the elements that come into contact with each other, and on the level of the pressure exerted axially by a pressure plate which presses the friction elements together.
In order to increase the axial force exerted on the engaging friction surfaces of the friction elements in the above-mentioned friction clutch, and hence the torque transmitted by the friction clutch, two pressure-generating clutch elements located farthest from the pressure plate have matching opposed ramp portions on their facing surfaces, disposed concentrically with respect to the axis of rotation of the clutch. The ramp-like surface regions of these two pressure-generating elements are so configured and arranged that the two elements are forced axially apart upon relative rotation by the engaging ramp-like surfaces so that an additional axial force is exerted on the friction elements, adding to the axial force applied by the pressure plate itself.
At the same time, the pressure plate of this conventional friction clutch, just as in the case of other conventional friction clutches of this type having no engaging ramp surfaces for increasing the axial force, must traverse a given actuation distance before the clutch becomes fully engaged. The extent of this actuation distance depends on the elasticity of the assembly of friction elements and on the total clutch spacing, i.e., the total of the separate axial spaces between the individual friction elements when the clutch is not actuated. Thus, the greater the total number of friction elements, the greater the actuation distance. As the friction linings undergo wear, the total clutch spacing increases correspondingly, so that the actuation distance becomes even greater.
In many cases, however, greater actuation distances, or actuation distances which increase markedly during operation because of lining wear, are not acceptable. For example, limited space conditions may permit only a limited actuation distance, or the time behavior of clutch performance may be unduly impaired by increased actuation distance.
Examples of such critical applications are motor vehicle servo steering gears of the type disclosed, for example, in German Offenlegungsschrift No. 36 12 619, in which the servo moment assisting the steering force is supplied not by a hydraulic cylinder and piston system but by a rotary, preferably electric, servo motor. This moment is transmitted by way of at least one friction clutch to the shaft of the steering gear. In such a motor vehicle servo steering gear, the actuation distance of the friction clutch employed must be comparatively small and must remain as small as possible because the servo assistance must begin after a very small angle of motion.